Add ipmi plugin

[ebookbug]

This plugin wants to collect the sensor metrics of bare metal.

Unlike the sensor plugin,the ipmi plugin will collect remote sensor metrics using ipmitool in linux

One plugin can collect multiple bare metal hosts.

[sparrc]

Overall looks good, thank you @ebookbug for the contribution, but two things I will need to merge:

1. Write unit tests, you can probably do this by modifying your code to make the IpmiCommand.Run function "pluggable", see the exec plugin for a good example of this in practice: https://github.com/influxdata/telegraf/tree/master/plugins/inputs/exec
2. Write a README in this format: https://github.com/influxdata/telegraf/blob/master/plugins/inputs/EXAMPLE_README.md

[ebookbug]

OK,finished.
Pls check it.

[sparrc]

Please change "host" to "server" (so that we don't override the builtin "host" tag key)

[ebookbug]

The default host tag will use the local hostname which the telegraf agent running on,but ipmi plugin collect remote bare metal host's metric.
I think the local hostname is not usable,but a more tag will increase the series cardinality.

[sparrc]

there is now an option to disable the default hostname, please change host to server

[ebookbug]

The tags have changed to 'server','name'.
Please check it.